a) Field of the Invention
The present invention relates to a shutter blades drive mechanism for photographic cameras which functions to drive shutter blades by using a motor comprising a magnet rotor allowed to rotate only within a predetermined angular range in a normal direction or another direction reverse thereto dependently on a direction of an electric current supplied to a coil.
b) Description of the Prior Art
As exemplified by the shutter blades drive mechanisms disclosed by U.S. Pat. No. 5,258,797 and Japanese Utility Model Preliminary Publication No. Hei 3-71328, there is known a shutter blades drive mechanism comprising a driving pin which is structured integrally with the magnet rotor and rotates together with the magnet rotor in the normal direction or the reverse direction; a shutter blades opening and closing lever which is biased by a spring in a direction to open the shutter blades; and a latch lever which engages with the shutter blades opening and closing lever at a position to close the shutter blades.
The shutter blades drive mechanism of such a type is configured so as to perform an operation for opening the shutter blades by disengaging the latch lever when the driving pin is shifted in the normal direction and another operation for closing the shutter blades by pushing the shutter blades opening and closing lever when the driving pin is shifted in the reverse direction. In a condition where the shutter blades are closed, the magnet rotor maintains itself at a stationary position with a magnetic force of itself. Therefore, the shutter blades drive mechanism of this type poses a problem that it allows the magnet rotor to be rotated until the latch lever is pushed by the driving pin when an impact is applied to a camera, thereby causing the disengagement of the shutter blades opening and closing lever.
Further, there is known another shutter blades drive mechanism which is of the type described above, but in which the shutter blades opening and closing lever is biased by a spring in a direction to close the shutter blades. In case of this shutter blades drive mechanism, however, the driving pin charges the spring at a process to open the shutter blades. Accordingly, the operation for opening the shutter blades is easily influenced by the spring when it has high resilience, whereby a time required of the operation for opening the shutter blades becomes unstable. Since an exposure time control circuit is triggered at an initial stage of the operation for opening the shutter blades in order to control a time until the operation for closing the shutter blades start, the instability of the time required of the operation for opening the shutter blades makes it impossible to precisely control an exposure time.